This invention relates to a reinforced composite structure adapted for use in a wide variety of sealing and load transfer applications and to a method and apparatus for making same.
Thin wall sheet material is oftentimes corrugated to achieve an increase in its cross-sectional area and section modulus without increasing the section wall thickness. Corrugations in sheet material normally serve to increase the edge compression strength as well as the stiffness and beam strength of a panel or tube into which the sheet is configured. Conventional corrugated material used for pipe, panel and planking exhibit a cross-section having a uniform undulating concave and convex pattern. The corrugated pitch and depth of corrugated sheet material are governed by design criteria seeking the greatest section modulus from sheet material having a fixed wall thickness.
These criteria are satisfactory when the end use of conventional corrugated sheet material is principally one requiring stiffness and compression strength to resist deflection and buckling. Such conventional corrugated cross-sections are found less satisfactory when used to resist pressure, shear and tensile loads and to transfer such loads to adjacent structure. Therefore, design criteria, other than those which provide maximum section modulus for a given wall thickness, must be considered to obtain maximum strength and load transfer capability of corrugated plastic laminate material used as primary structure in tanks, tubes and joints.
It is well known in the art that when a flexible cable is suspended between two points and is uniformly loaded across the span bridged by the cable, the horizontal component of the resulting cable tension remains constant along the cable length. If the expression W/T represents the constant ratio of the vertical component of load per unit of cable length W to the horizontal component of cable tension T, then the expression y=x.sup.2 W/2 T represents the equation of a parabolic curve into which the cable is deflected by the uniform span load if the lowest point of the cable is chosen as the origin of the coordinate system.
I have discovered that when a ply of continuous filament strands is uniformly pressed upon an underlying ply of filament strands, supported to bridge a forming gap, that the strands of the two plies are compacted uniformly. Such compaction functions to deflect the plies into a generally concave parabolic configuration in the forming gap into which the plies are deflected. When the plies are impregnated with and bonded together by an adhesive, such as a polymeric resin, the resulting composite structure will exhibit enhanced structural properties heretofore unobtainable in the types of structures under consideration.
Prior art methods for fabricating and compacting continuous filament strands contained in plastic laminates generally involve tensioning individual strands of warps or the passing of resin-impregnated filament strands through orifices or gaps configured to remove surplus resin from individual filaments. Such mechanical compacting and resin removal methods tend to break or otherwise damage substantial quantitites of filaments, normally composed of brittle materials, such as glass.
Prior art forming methods and apparatus for fabricating corrugated plastic laminates generally involve the use of corrugating forms having continuous forming surfaces maintained in full contact with the formed laminate. Single-face vacuum bag molds, two-part matched die pressure molds and roller in groove type devices are typical of such conventional apparatus. The formed corrugated laminate is normally designed solely to produce sections which exhibit the greatest stiffness for the least laminate thickness and are satisfactory for fabricating laminate intended for use in secondary load applications, such as well panels, fencing and lightly loaded roofs.
Such methods and apparatus are unsatisfactory for fabricating corrugated plastic laminates intended for use as primary load bearing structure required to resist the transfer shearing and tensile loads. In addition, conventional corrugated plastic laminates oftentimes comprise polymerized mixtures of chopped glass fiber and resin combined with one or more layers of resin-impregnated woven glass fabric. Heretofore, corrugated laminates of this type have been expensive and difficult to fabricate and when fabricated, oftentimes exhibit relatively thick walls.